Introduction to Perl

1
1.0.1.8.1 Introduction to Perl Session 1

• what is Perl?
• history of Perl
• writing and running a Perl script
• dealing with variables

2
what is Perl?

• Perl is a programming language (duh)
• Perl has a philosophy of pragmatism, creativity
  and fun
• it lets you get the job done
• it makes easy jobs easy and hard jobs possible
• it makes it easy to manipulate numbers, text,
  files, strings, directories and processes
• its free, available on nearly every platform
• theres more than one way to do it
• its simple to learn but deep enough to continue
  to stimulate for years to come
• its highly idiomatic just like a language
• it works on the principle of least surprise
• its remarkably extensible (Comprehensive Perl
  Archive Network)
• Perl is extremely rich just like a language
• you can pick up conversational Perl in a few
  weeks
• you can write Perl poetry in a few months
• you will speak Perl slang shortly after that

3
history of Perl

• 1987 Perl 1.0 is released by Larry Wall
• practical extraction and report language
• pract
• 1988 Perl 2.0 is released
• sort operator added, among other things
• 1989 Perl 3.0 is released
• you can now pass things by reference, among other
  things
• 1991 Perl 4.0 is released
• I'm reminded of the day my daughter came in,
  looked over my shoulder at some Perl 4 code, and
  said, "What is that, swearing?" Larry Wall
• 1995 Perl 5.0 is released
• POD is introduced, among other things
• Perl 6.0 will be released when its ready
  current version 5.10.0 (13-may-08)
• named parameters will be added, among other things

Larry Wall
http//history.perl.org/PerlTimeline.html
http//dev.perl.org/perl6/faq.html
4
Myths of Perl

• Perl looks like line noise
• Perl is hard because
• it has regexps
• it has references
• Perl is just for UNIX
• Perl is just for one-liners you cant build
  real programs with it
• Perl is just for the web
• Perl is too slow
• Perl is insecure

http//www.perl.com/pub/a/2000/01/10PerlMyths.html
5
a few notes before we begin

• Perl is a practical alternative to bashing your
  head against the wall
• anyone can learn Perl and make good use of it
• for every 1 hour learning Perl you will save 1
  month of work
• train your eyes to quickly spot the difference
  between
• whatever we are doing, thinking whether there is
  another way to do it
• Perl gives you a lot of freedom control
  yourself!
• we wont write Hello World in Perl, but you can
  see it in Perl and many other languages at
  http//www.freenetpages.co.uk/hp/alan.gauld/compla
  ng.htm

()
6
running your first Perl script

• Perl is interpreted
• you dont need to compile your scripts
• a variety of Perl binaries exist on our system
• /usr/local/bin/perl (5.005)
• /usr/local/bin/perl56 (5.6.1)
• /usr/local/bin/perl58 (5.8.3)
• /home/martink/perl/current/bin/perl (5.8.7)
• to check version perl V head -1
• Summary of my perl5 (revision 5.0 version 8
  subversion 3) configuration
• executing a Perl script is easy just pass the
  file to the Perl binary

gt cat script.pl print Camels spit up to 10
meters, except in the US where they spit up to 33
feet.\n gt /usr/bin/perl script.pl Camels spit
up to 10 meters, except in the US where they spit
up to 33 feet.
7
! notation

• you can specify the Perl binary within the script
• this is the preferred way of doing it
• you can pass flags to the Perl binary, if needed
• well cover useful flags later

!/usr/bin/perl print I saw a smoking camel.\n
!/usr/bin/perl -w print I saw a smoking
camel.\n
http//sunsite.uakom.sk/sunworldonline/swol-09-199
9/swol-09-unix101.html
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setting executable flag

• your shell will automatically execute binaries
  if their executable flag is set
• iterative script writing process
• create/edit your script with your favourite text
  editor
• set executable flag on with chmod (once)
• run/debug script

gt ls -rw-r--r-- 1 martink users 112
2006-04-04 1258 script.pl gt chmod x
script.pl -rwxr-xr-x 1 martink users
112 2006-04-04 1258 script.pl gt which
script.pl ./script.pl gt script.pl Camels spit up
to 10 meters, except in the US where they spit up
to 33 feet.
9
choice of perl binaries

• on any large network, you will find many versions
  of the Perl interpreter (perl)
• /usr/bin/perl installed with the OS on the
  network node
• /usr/local/bin/perl installed for system-wide
  use, long long ago
• /usr/local/bin/perlxxx variety of links to
  other perl versions
• if you would like to play around
• /usr/bin/perl
• if you are just starting and have no legacy
  dependancies
• /usr/local/bin/perl58
• additional modules may have been installed by
  systems
• if you need perl 5.6 for legacy use
• /usr/local/bin/perl56

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checking for version and binary compile settings
gt /home/martink/bin/perl V Summary of my perl5
(revision 5 version 8 subversion 7)
configuration Platform osnamelinux,
osvers2.4.20-64gb-smp, archnamei686-linux-ld
uname'linux xhost02 2.4.20-64gb-smp 1 smp wed
aug 6 183002 utc 2003 i686 unknown unknown
gnulinux ' config_args'' lt...gt
Compiler lt...gt intsize4, longsize4,
ptrsize4, doublesize8, byteorder1234
d_longlongdefine, longlongsize8,
d_longdbldefine, longdblsize12
ivtype'long', ivsize4, nvtype'long double',
nvsize12, Off_t'off_t', lseeksize8
alignbytes4, prototypedefine Linker and
Libraries lt...gt Dynamic Linking
lt...gt Characteristics of this binary (from
libperl) Compile-time options USE_LONG_DOUBLE
USE_LARGE_FILES Built under linux Compiled at
Sep 20 2005 161946 _at_INC
/home/martink/perl/5.8.7/lib/5.8.7/i686-linux-ld
/home/martink/perl/5.8.7/lib/5.8.7
/home/martink/perl/5.8.7/lib/site_perl/5.8.7/i686-
linux-ld /home/martink/perl/5.8.7/lib/site_per
l/5.8.7 /home/martink/perl/5.8.7/lib/site_perl
.
11
Perl variables

• Perl does not require that you specify what you
  want to store in a variable
• this is a contrast to typed languages like C or
  Java
• this is a boon and a bane but you are in
  control, not the language
• the same variable, at different times, can hold
• a number
• a string
• a letter
• binary data
• Perl differentiates variables on the basis of
  plurality
• a scalar variable holds a single value (a number,
  a string, a letter)
• an array variable holds multiple values (a list
  of numbers, a list of strings)
• a hash is a special type of array variable in
  which elements are indexed by strings, not
  integers

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Perl variables

• Perl variables are preceded by a character that
  identifies the plurality of the variable
• you cannot access the value in a variable without
  using the appropriate prefix
• animal, _at_animal and animal are different
  variables
• they are completely independent
• they can hold different values

animal
animal
_at_animal
animal
scalar
array
hash
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Perl variables

• you can name your variable whatever you want
  (mostly)
• no special characters like _at_ in variable names
  (obviously)
• cannot begin with a number
• no spaces
• Perl is case sensitive
• animal Animal and ANIMAL are all different
  variables

good bad animal animal123 123animal big_an
imal big animal BigAnimal biganimal
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Variable Assignment

• to give a variable a value, use
• use undef to force a variable to become undefined

x 1 y 2 z 3 x y 0 set both
variables to zero (x,y,z) (1,2,3) well
see this later
x 1 x undef explicitly undefines y
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Scalars

• scalars are identified by and can hold only one
  value at a time
• scalar is like a cup if you want coffee, you
  need to remove the tea
• arrays are more like icecube trays you can have
  many icecubes

!/usr/bin/perl a comment animal
Camel print animal,\n animal 12
Camels print animal,\n animal 12 an
inline comment print animal,\n Camel 12
Camels 12
1. always specify where your perl interpreter is
2. each line must be terminated by a semicolon
3. \n codes for new line
4. indicates the rest of the line is a comment
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Basic Operators

• Perl has a lot of different operators actions
  that you can apply to variables
• unary/binary/trinary operate on one/two/three
  scalars at a time
• Perl will try to do the right thing when you are
  operating on scalars
• try mixing numbers and strings in an operation to
  see what happens

w camel x 2 y 3 z x y
5 z x y 6 z x / y 0.66666 z
w x 2 z w x 0 z w . x
camel2 z x . y 23 z w w 0
- / basic arithmetic 23
5 exponentiation 23 8 . concatenation
(period) 2.3 23
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Functions

• functions are things that Perl knows how to do
  out of the box
• sqrt( )
• sin( )
• rand( ) I will use rand() for a lot of examples
• you can write your own functions, of course

x rand() x is a random number uniformly
sampled from 0,1) print x,\n print
sqrt(x),\n print xx,\n 0.0730786472558
975 0.27033062581938 0.825975844619357
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Flow Control

• Perl has a wide variety of branching operators
• looping
• condition checking
• lets learn the if conditional so we can write
  simple scripts
• conditional operators
• tests for equality between numbers
• eq tests for equality between strings

x rand() if ( x lt 0.5 ) print x .
is small\n else print x . is
large\n
if ( CONDITION ) CODE else CODE
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Many Perlisms Ahead

• Perl is about doing the same thing in a variety
  of ways
• be creative
• be stylish
• be careful!
• start slowly and increase flair as necessary
• make sure that, above else, you can understand
  your code!

if ( x lt 0.5 ) print x,\n if ( x lt
0.5 ) print x,\n print x,\n if x lt
0.5
if ( CONDITION ) CODE
CODE if CONDITION
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Interpolation

• interpolation can be a great source of
  frustration
• Perl tries to make it as painless as possible
• how a language interpolates variables is how a
  language decides how to evaluate strings, which
  may contain variables
• rule 1 Perl interpolates variable values in
  double quotes

x Camel y I have a pet x print
y I have a pet Camel
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Interpolation double quotes

• you can safely tuck your variables inside double
  quote and their values will be evaluated and
  inserted into the string
• variables will be interpolated, but no operations
  will be performed

x Camel y1 I have a pet x y2 I
have a pet . x y3 I have a pet . x
x Camel y 2 z 3 w I have a pet
x who told me y times z is yz I have a
pet Camel who told me 2 times 3 is 23
22
Interpolation double quotes

• if you want results of operations included in
  strings
• concatenate them in
• use temporary variables

x Camel y 2 z 3 t y z w1
I have a pet x who told me y times z is
t w2 I have a pet x who told me y times
z is . t w2 I have a pet x who told me
y times z is . y z I have a pet Camel
who told me 2 times 3 is 6
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Interpolation single quotes

• no interpolation happens if you use single quotes
• x is a string that contains the characters
  and x, not the variable x
• you may want to print the text x and not the
  value of the scalar x

x Camel y 2 z 3 t y z w1
I have a pet x who told me y times z is
t I have a pet x who told me y times z is
t
s1 x s3 . x s2 \ . x
since is a special character, it needs to be
escaped in double quotes
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Interpolation an ation of pain

• youll get used to Perls own interpolation
  mechanism, but at first it can be frustrating
• you have just discovered the mysteries of Perls
  special variables
• special, as in hidden and confusing and
  impossible to remember
• dont worry, well get to these shortly
• for now, if you have words or numbers preceeded
  by or _at_ or in your strings, expect the
  unexpected!
• dont worry, well sort these things out
  eventually

x If I join the espresso club, I will save 2
on every coffee! If I join the espresso club,
I will save on every coffee!
whats going on? wheres your money?
25
Interpolation

• understanding and getting a handle on
  interpolation is important because youll be
  wanting to print things out
• Perl offers assistance in interpolating your
  strings
• think of the quotes as an operator, not as a
  container for a string
• operates to replace all mention of variables
  with their values
• operates to ignore all mention of variables
  and treats the string as a literal
• instead of quotes, you can use quote and
  quote-like operators

x camel print x camel print
qq(camel) camel print qq(x)
camel print qq(x) camel
qq(STRING) is equivalent to STRING
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Uses of qq( )

• qq( ) helps you deal with strings which have
  quotes in them
• remember, the qq( and ) are the parts of the
  operator.

x My camels name is Bob print
x Bareword found where operator expected at
./script.pl line 21, near ""My camel's name is
"Bob" (Missing operator before Bob?) syntax error
at ./script.pl line 21, near ""My camel's name is
"Bob" String found where operator expected at
./script.pl line 21, near "Bob""" Execution of
./script.pl aborted due to compilation errors.
x1 My camels name is \Bob\ x2 qqMy
camels name is Bob
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Flexibility of qq( )

• remember how I said Perl is flexible and gives
  you control
• how about flexible delimiters? now thats
  control!
• non alpha-numeric, non whitespace
• you get the idea
• there are other operators that have this
  flexibility
• pick a delimiter and stick with it

qq(My camels name is Bob) qqMy camels name
is Bob qq/My camels name is Bob/ qqMy
camels name is Bob qqMy camels name is
Bob qqMy camels name is Bob qq!My
camels name is Bob!
qq/My camels name is Bob. His answer to x is
spitting and his favourite char is (/
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qq( ) vs q( )

• q( ) is equivalent to single quotes
• all the flexibility of qq( ) without the
  interpolation
• if you have strings with lots of special
  characters, qq() and q() are a boon

x 2 q(x) x qq(x) 2
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VAR

• consider the following problem
• you want to print out a variable and immediately
  another string after it

x 10 print qq(Camels spit up to xm)
bad no variable xm print qq(Camels spit up to
xm) good print Camels spit up to xm
good print Camels spit up to x.m
good but messy print Camels spit up to x \bm
obfuscated
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Interpolation Examples

• lets apply some of the things weve seen

x x X X print q(x) . qq( is the
string x) print q(X) . qq( is the string
X) x is the string x X is the string
X if ( x eq X ) print qq(\x x and
\X X have the same contents) else
print qq(\x x and \X X are
different) x x and X X are
different print qq(If I get a new camel, I will
name him x.X.x) If I get a new camel, I will
name him x.X.x
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Contratulations you have conquered your fears

• you can now understand the following Perl line
  noise see its not that hard

x 1 x. 1. x.. 1. qq(x).. 1. qq(
x.) 1. qq(x.) 1. q!x! x \xx.
x1. xxx x1x \xxx x1x qq(xx1
\x) 111x
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1.0.8.1.1 Introduction to Perl Session 1

• you now know
• all about scalars
• all about interpolation
• qq( ) and q( )
• and eq
• if conditional
• next time
• manipulating strings
• regular expression basics